support resuming downloads

[StefanKarpinski]

https://discourse.julialang.org/t/install-binary-package-such-as-openblas-jll-via-very-slow-internet-connection/36335

It would be a nice feature to allow resuming downloads by saving the partial download somewhere somewhat persistent if the client and server sides both support HTTP range requests. Kind of a fancy feature for the future, but it would be a really nice one for people on lousy connections.